Just to add to the suggestions about finding information on the effectiveness of the wearing of school uniforms, you may also wish to interview the administrators of Catholic schools since these schools have had uniforms as a requirement for a long time. Also, consider using the testimony of a former parochial school student who can provide you with a good quote for your introduction. Here is one comment made by an adult who attended parochial school:

Although we had days on which we could wear nice skirts or dresses, I really don't recall what other girls wore. Instead, I remember faces and hairdos and things that the students said and did. My memories of their looks are just like their pictures in the yearbook: merely busts of them. I think because we wore uniforms, we got to know the person better and did not judge her by her appearance so much. Anyway, we all seemed to be on a even plane with one another since no one looked better or richer than the others.

Statistics are important in your argument, but testimony from real people in your area will be convincing. Good luck!

I think that the previous post did a great job of setting up the structure of your essay. In terms of finding reliable sources, I think you have to start off with the internet. A simple web search of "pros and cons of school uniforms" will render a great deal. As with all internet research, you will have to find not just the information, but actually the best samples of what is out there. You might want to use reference websites to a great extent. There are plenty of websites that feature both sides of a potential argument. This would allow you to not only present a convincing side, but be able to integrate some potential arguments against what you are advocating in your writing, making it that much stronger in preempting and being mindful of what another would say against your analysis or evidence. The web is a great resource for this. Another angle might be the personal interview, where you ask different people their opinion about the topic. This might not be as entirely objective or reliable because of its personal nature, but I think it might be able to bring a personalized insight to your writing.

1. Brainstorm a list of reasons you think students should wear uniforms. Start with a bunch of ideas that are all yours. Then, research, and add examples of real facts you found that support your first ideas.